Near Resonance vibronic coupling. Isoquinoline

The interaction spectrum is considered resulting from the near-resonance coupling of a small number of vibronic levels of two different electronic states. In the simple model proposed, the interactions between the near-resonance states are first explicitly considered and then the off-resonance interactions are treated by perturbation theory. The model is applied to the isoquinoline spectrum and it is shown that for isoquinoline the latter interactions may be safely ignored. Good agreement is achieved between the theoretical and experimental spectra, and many puzzling features, such as the irregular nature of the sequence structure and the ambiguous isotope effects are readily explained. The lowest excited singlet state is the nπ*, and it is located within about 1000 cm^?1 of the first excited ¹ππ*. Two vibronic levels of the ¹nπ* state, corresponding to single quanta of the out-of-plane vibrations, are in near-resonance with the vibrationless ¹ππ* level.